The invention relates to a projection display device comprising an illumination system having a light source and an optical guiding means having an entrance surface for entering light from the light source and an exit surface for providing an illumination beam, an image display system having a display panel for modulating the illumination beam with image information and projecting the image on a screen, and means for selectively transmitting a portion of the illumination beam having a desired characteristic to the image display system and means for feeding back a portion of the illumination beam having an undesired characteristic to the illumination system for re-illumination of the image display system.
Projection display devices can be used in both rear and front image projection systems. In a rear projection system, the projection display device projects an image representing television or datagraphic information on the rear side of a diffusing transparent screen, which front side is directed to a viewing audience. In a front projection system, the projection display device projects an image representing television or datagraphic information on the front side of a reflecting screen, which front side is directed to a viewing audience.
Such a projection display device is known from the published international patent application WO98/23990. The known projection system comprises a light source and an integrating system for producing an illumination beam and an image display system having an image display panel for modulating the illumination beam with image information and projection of the modulated light beam on a screen. The integrator system has an exit surface which is located in an exit plane and a reflector present in the exit plane. Furthermore, the display device comprises a selective mirror and an optical system for partly imaging the exit aperture of the integrator system via the second selective mirror on the first reflector. In this arrangement, a portion of the illumination beam, which is not used for image formation, is fed back to the illumination system, so that at least a part of the reflected portion can be reused. The known system has the drawback that the optical system can be costly and must be adjusted during manufacture.
It is an object of the invention to provide a projection display device which is easy to manufacture and has a reduced number of optical components in the illumination system. This object is achieved by the projection display device according to the invention, and is characterized in that a reflection means is provided at a portion of the entrance surface of the light-guiding means and the unused portion of the illumination beam is fed back into the light-guiding means. In the device as claimed, the unused portion of the illumination beam may be, for example, the portion of the illumination beam which has not the desired polarization or desired color or the portion of the illumination beam which is not modulated by the display panel because it reflects from the partially reflective grid between the picture elements of the display panel or, if the illumination beam overfills the display panel, the portion of the illumination beam which is not modulated by the display panel because it falls outside the display panel. The recuperated light is then reflected by the reflecting means at the entrance surface of the light-guiding means, redistributed by the light-guiding means and mixed with the light from the light source.
In a particularly advantageous embodiment of the invention, the light-guiding means comprises a rod of optically transparent material. This optically transparent rod provides a homogeneous distribution across a cross-section of the illumination beam perpendicular to the propagation direction of the illumination beam.
In a further embodiment of the projection display device according to the invention, the unused portion of the illumination beam is fed back into the light-guiding means via its exit surface. This arrangement allows a compact design of the illumination system.
A further embodiment of the projection display device according to the invention is characterized in that the reflection means is a reflecting layer provided at the portion of the entrance surface of the light-guiding means. The size of the portion in the entrance surface determines the optical efficiency because the light can leave the optical system only via this portion or via the active areas of the display panel. If the illumination beam of the lamp is not homogeneous, the light distribution can be improved by dividing the reflecting layer of the entrance surface into a first portion and a second portion arranged around the first portion and separated from the first portion by an uncovered portion of the entrance surface.
In a further embodiment of the projection display device according to the invention, the means for selectively transmitting a desired portion of the illumination beam comprises a reflective polarizer or a dichroic filter. In a further embodiment of the projection display device according to the invention, the color filters are patterned on a wheel and each of the red, green or blue filters occupies a spiral-shaped portion of the wheel. The wheel is rotatably mounted and provided in operation with an illumination beam having a red, a green and a blue line-shaped portion for simultaneously scanning the display device.
A further embodiment of the projection display device according to the invention is characterized in that the projection display device is provided with a transmissive image display panel or a reflective image display panel. The transmissive image display panel may be a transmissive liquid crystal display. The reflective display panel may be a digital micromirrored display (DMD) panel or a reflective liquid crystal on silicon (LCOS) display panel.
In another embodiment of the projection display device according to the invention, a further light-guiding means is provided between the exit aperture of the light-guiding means and the image display panel for guiding the illumination beam to the image display panel. The means for selectively transmitting a desired portion of the illumination beam and reflecting an undesired portion of the illumination beam can be positioned between the exit aperture and the further light-guiding means or between the further light-guiding means and the image display panel. The further light-guiding means may comprise a prism of an optically transparent material or a system of lenses and mirrors.
Another embodiment of the projection display device according to the invention is characterized in that the projection display device comprises processing means for correcting colors of the projected image in dependence upon the color content of the image information. The color correction corrects for a higher intensity of a color in the illumination beam due to the recycling process.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.